Twice running speed frequency 1

[IKBrunel]

Hi all,

I am measuring bearing vibration and I have been told by a colleague that twice running speed responses are indicative of a bearing fault, particularly due to misalignment or inadequate preload.

The bearing is a deep groove ball bearing with a predominant axial load and runs at a very high speed. The 2x running speed response is much higher axially, seemingly due to our use of a radial damping component.

I can't understand why this response occurs. Please tell me if you know of any online references explaining why this is, or provide one yourself.

Many thanks

Matt

 

[Tmoose]

" 2X = misalignment, especially when axial is higher than radial" appears in many, if not most vibration analysis references. Here's one that is mostly a survey of other sources .

http://cp.literature.agilent.com/litweb/pdf/5962-7276E.pdf

It's "explanation" of a bent shaft causing 2X is arcane, but was widely held.

The cause/effect is not at all ironclad. For instance, some types of shaft couplings create 2X when misaligned, and some don't. I'd be unpacking the dial indicator and checking for mechanical runouts when referencing a good ground. Confirming Round holes and straight shafts must be the starting point.

My experience is mechanical looseness (rolling element and journal bearings) is very often accompanied by harmonics of 1X and truncated 1X wave form (same thing).

Bearing "faults" (individual spalls in races or balls" create impacts at frequencies related to dimensions, number of balls, and thus are related to rpm. Most are 3.xxx rpm or higher. Exactly 2X might be impossible.

 

[Strong]

2xSS does not indicate a bearing fault per se. But it can indicate that the dynamic load on the bearing is abnormal. If this bearing in in a spindle or similar machine type with preload or high thrust load, then I would investigate a cocked bearing (bearing races/rings not perpendicular to shaft axis) or a misaligned coupling between two shafts.

Walt

 

[FeX32]

Definitely not typical bearing fault.
"2X = misalignment" I agree.


Fe

 

[cuckooclock]

From a Hp Agilent brochure I quote

"The key distinguishing feature is a
high 2x component, especially in
the axial direction."

The cause is misalignment of the bearings. This brochure can be found at

http://cp.literature.agilent.com/litweb/pdf/5962-7276E.pdf

Title: Effective Machinery Measurements Using Dynamic Signal Analyzers (AN 243-1)

 

[hoyle]

Tmoose has the main point at which I would start - the passing frequency – the number of balls which pass underneath the load path. Not unheard of on heavily loaded bearings, in the radial direction. The inner race bends, then unbends dependent on there being a ball/roller there to support it. Picking it up clear enough to be definite at high speed may take some thinking about. Spectral analysers etc…
In the axial direction, the only one that makes sense to me having just read it now is basically a misalignment of the bearing in its housing. So it’s checking for dimensional accuracies and correctly machined bearing housing bores.
Let us know what you find out, I would be interested.